Method of locating conductive spheres utilizing screen and hopper of solder balls

ABSTRACT

Apparatus and methods for placing conductive spheres on prefluxed bond pads of a substrate using a stencil plate with a pattern of through-holes positioned over the bond pads. Conductive spheres are placed in the through-holes by a moving feed mechanism and the spheres drop through the through-holes onto the bond pads. In one embodiment, the feed mechanism is a sphere hopper which crosses the entire through-hole pattern. In another embodiment, a shuttle plate fed spheres from a reservoir and reversibly moves about one-half of the pitch, moving from a non-discharge position to a discharge position.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No.09/897,808, filed Jun. 29, 2001, pending, which is a continuation ofapplication Ser. No. 09/168,621, filed Oct. 8, 1998, now U.S. Pat. No.6,268,275, issued Jul. 31, 2001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to semiconductor devicemanufacturing. More particularly, the present invention is directed tomethods and apparatus for handling solder balls in forming ball gridarrays (BGAs).

[0004] 2. State of the Art

[0005] Integrated circuit semiconductor devices (ICs) are smallelectronic circuits formed on the surface of a wafer of semiconductormaterial such as silicon. The ICs are fabricated in plurality in waferform and tested by a probe to determine electronic characteristicsapplicable to the intended use of the ICs. The wafer is then subdividedinto discrete IC chips or semiconductor dice, and then further testedand assembled for customer use through various well-known individual ICdie testing and packaging techniques, including lead frame packaging,Chip-On-Board (COB) packaging, and flip-chip packaging (FCP). Dependingupon the semiconductor die and wafer sizes, each wafer is divided into afew dice or as many as several hundred or more than one thousanddiscrete die.

[0006] Interconnection of discrete semiconductor packages onto asubstrate such as a printed circuit board (PCB) is often accomplishedwith solder preforms having generally a spherical or other shape. In aprocess using a ball-grid-array (BGA), spherical solder balls areattached to prefluxed metallized locations on a workpiece such as acircuit board or a semiconductor device. The workpiece is then heated toreflow the solder balls, and the solder balls become attached to themetallized locations during subsequent cooling. A semiconductor packageor circuit board having a corresponding but reversed pattern ofconnection sites may then be aligned with the BGA and bonded to it bycontrolled heating in a reflow furnace.

[0007] The use of flip-chip technology with solder bumps has numerousadvantages for interconnection, it being widely used in the electronicsindustry. Flip-chip design provides improved electrical performance forhigh frequency processor applications, such as mainframes, computerworkstations, and personal computers having powerful processors.Ball-grid-array interconnections are of small size. In addition, easierthermal management and reduced susceptibility to EMI and RFI emissionsare inherent in the use of BGA technology.

[0008] In addition, surface mount technology (SMT) using solder “bump”or ball interconnects eliminates the outer package leads level ofinterconnection, significantly reducing the cost.

[0009] Solder bumps may be formed on a workpiece by processes ofevaporation, electroplating, stencil printing, and serial methods. Eachof these processes has particular limitations. U.S. Pat. No. 5,672,542of Schwiebert et al. is an example of a modified stencil printingprocess.

[0010] In U.S. Pat. No. 3,716,907 of Anderson, the use of germaniumhemispheres as conductive contacts is disclosed. The germaniumhemispheres are connected to the substrates with solder.

[0011] Relative to other types of interconnections, the use of solderpreforms, in particular spherical or near-spherical balls, has proven tohave significant advantages. One advantage is that while the solderballs are formed with ball-to-ball size differences, they may be easilyclassified by size prior to application to a workpiece. Thus, a uniformsize of solder balls may be used within a ball-grid-array.

[0012] Various methods have been used for aligning, placing, retainingand fixing solder balls on an array of sites on a workpiece.

[0013] In U.S. Pat. No. 5,620,927 of Lee, a template with an array ofthrough-holes is placed on the workpiece and solder balls are introducedinto the holes by rolling the solder balls across the workpiece surface.The apparatus may be installed on a tilt table to encourage filling ofall holes. In U.S. Pat. No. 4,871,110 of Fukasawa et al., a templatehaving an array of holes is placed on a ball holder with a like array ofsmaller holes to which vacuum is applied and over which solder balls arerolled. After the array is filled with solder balls, the template andball holder with balls are removed and the exposed ends of the ballsattached to a substrate by e.g. reflow. The template and ball holder arethen pulled from the substrate, leaving a ball-grid-array ready forattachment to another substrate or workpiece. A vacuum system isrequired, and there is no easy way to replace a solder ball onto a bondpad to which a ball did not become attached (i.e., missing ball).

[0014] As shown in U.S. Pat. No. 3,719,981, an array of solder balls isarranged on the tacky surface of a pressure sensitive (PS) tape foralignment through a template to solder bumps on a wafer. After thermalreflow, the template and tape are removed.

[0015] The use of a template for forming solder bumps or “balls” on aworkpiece from flux and solder pieces is disclosed in U.S. Pat. No.5,492,266 of Hoebener et al.

[0016] In U.S. Pat. No. 5,431,332 of Kirby et al., a template is placedover the bond pads of a substrate, solder balls are poured over thetemplate, and an air knife “sweeps” the surface free of excess solderballs.

[0017] The use of a ball pick-up tool with an array of vacuum suctionball retainers to pull up balls from an underlying reservoir and placethem on a substrate is disclosed in U.S. Pat. No. 5,088,639 of Gondotraet al., U.S. Pat. No. 5,284,287 of Wilson et al., U.S. Pat. No.5,445,313 of Boyd et al., U.S. Pat. No. 5,467,913 of Nemekawa et al.,U.S. Pat. No. 5,615,823 of Noda et al., U.S. Pat. No. 5,680,984 ofSakemi, U.S. Pat. No. 5,685,477 of Mallik et al., U.S. Pat. No.5,687,901 of Hoshiba et al., and U.S. Pat. No. 5,695,667 of Eguchi etal. It is known in the art that shutting off the vacuum to release eachball onto the substrate is not always successful, and sometimes ballsremain attached to the pick-up tool. Again, there is no easy way toreplace a missing ball except with a single ball pickup tool.

[0018] U.S. Pat. No. 5,506,385 of Murakami et al. discloses the use of asingle manipulable suction head for picking up a solder ball, moving itto a position above a fluxed contact pad on a substrate, and depositingit on the contact pad. Because of the high number of repetitive actionsin separate placement of each ball, ball placement is time consuming.

[0019] U.S. Pat. No. 5,695,667 shows a single ball suction head which isused to place a solder ball on a contact pad which is missing a solderball of a ball-grid-array.

[0020] The application of flux to solder balls held in a vacuumapparatus by dipping the balls into a flux reservoir is taught in U.S.Pat. No. 5,088,639 of Gondotra et al. and in U.S. Pat. No. 5,284,287 ofWilson et al.

[0021] The use of ultrasonic vibration to cause solder ball movement inthe ball reservoir, and to remove excess solder balls from a vacuumpickup tool, is taught in U.S. Pat. No. 5,687,901 of Hoshiba et al.

BRIEF SUMMARY OF THE INVENTION

[0022] The invention comprises apparatus and methods for rapidly,accurately, and reliably placing an array of conductive spheres such assolder balls on conductive sites, e.g. bond pads, on a substrate. Thesubstrate may be a circuit board of any composition, e.g. BT resin, ormay be a silicon wafer or even a single semiconductor die such as an “ICchip”. The conductive sites on the substrate may comprise bond padswhich include those which project from the substrate and those which arerecessed into the substrate surface. Projecting bond pads require apre-application of flux or other sticky substance by which the spherescling to the bond pads. Use of flux or sticky substance may notnecessarily be required with recessed bond pads.

[0023] The apparatus includes a stencil plate or screen overlying thesubstrate, wherein the stencil plate is parallel to and slightly spacedfrom the substrate. The stencil plate has an array of through-holescorresponding to a desired placement pattern of conductive spheres onthe substrate. The invention also includes ball supply apparatus forproviding conductive spheres to the stencil plate, wherein allthrough-holes in the stencil plate are filled with one, and only one,sphere. Spheres placed into the through-holes of the stencil plate dropby gravity to the substrate for retention by pre-applied flux or bydepressed bond pads. Each through-hole is slightly larger than a sphereand constrains a sphere on the substrate until the substrate and stencilplate are further separated e.g. for solder reflow. The stencil platethickness and proximity to the substrate prevent more than one ball fromentering each through-hole of the stencil plate.

[0024] A first embodiment of a ball supplying apparatus is asphere-retaining hopper with a lower opening through which spheres maydrop into through-holes of the stencil plate and thence onto thesubstrate surface. The hopper is closely spaced from the stencil plateto maintain control over all the spheres therein. Sphere placement isaccomplished by horizontal movement of the hopper across thethrough-hole pattern of the stencil plate, filling each through-holewith one, and only one, sphere. As the hopper moves, only the spheresdropping into the through-holes, one to a through-hole, can escape fromthe hopper. The numbers of spheres passing over each through-hole ensurethat each hole is filled, but a higher degree of assurance can beobtained by making several passes.

[0025] In a second embodiment, a sphere supply apparatus includes ashuttle plate with the same through-hole pattern as the stencil plate.The shuttle plate closely overlies the stencil plate and is reversiblymovable between a first position wherein its through-hole pattern isaligned with the pattern of the stencil plate and a second positionwherein the through-hole patterns are non-aligned. In the firstposition, spheres may drop from the shuttle plate through-holes into thestencil plate through-holes. In the latter position, spheres areprevented from entering the through-holes of the stencil plate. Thethrough-holes of the shuttle plate may be fed from an overlying openbottom reservoir, which may be fixed to the shuttle plate or fixed inposition. The linear movement of the shuttle plate is less than theinter-sphere distance, i.e. pitch, and is generally equal to aboutone-half of the pitch.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0026] The following drawings illustrate various embodiments of theinvention, not necessarily drawn to scale, wherein:

[0027]FIG. 1 is a perspective exploded view of exemplary apparatus ofthe invention for placing conductive spheres on a substrate;

[0028]FIG. 2 is a sectional side view of a substrate and exemplaryscreen for applying flux to the bond pads in a step of a method of theinvention for placing conductive spheres on a substrate;

[0029]FIG. 3 is a sectional side view of a stencil fixture shown inoverlying relationship to a prefluxed substrate ready to receiveconductive spheres in a step of a method of the invention for placingconductive spheres on a substrate;

[0030]FIG. 4 is a sectional side view of a sphere placement apparatus ofthe invention showing spheres placed on the bond pads of a substrate, astaken along line 4-4 of FIG. 1;

[0031]FIG. 5 is a sectional side view of a substrate having conductivespheres placed on the bond pads of the substrate in accordance with asphere placement method of the invention;

[0032]FIG. 6 is a sectional side view of a substrate having conductivespheres placed on the bond pads of the substrate and reflowed inaccordance with a method of the invention;

[0033]FIG. 7 is a partial sectional side view of a stencil fixture ofthe invention;

[0034]FIG. 8 is a partial sectional side view of another embodiment of astencil fixture of the invention;

[0035]FIG. 9 is a partial sectional side view of a substrate withrecessed bond pads having conductive spheres placed thereon, inaccordance with a sphere placement method of the invention;

[0036]FIG. 10 is a cross-sectional end view of another embodiment of ahopper of the invention, as taken along line 10-10 of FIG. 1;

[0037]FIG. 11 is a perspective exploded view of another embodiment ofthe invention for placing conductive spheres on a substrate;

[0038]FIG. 12 is a sectional side view of another embodiment of anapparatus for placing conductive spheres on a substrate, shown in apreplacement step in a method of the invention; and

[0039]FIG. 13 is a sectional side view of a sphere placement step in amethod of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0040] The invention comprises an improved method and apparatus forplacing a plurality of conductive spheres 12, such as preformed solderballs or germanium balls, on conductive sites 14 on a surface 16 of asubstrate 20. The term “substrate” is used in a broad generic senseherein to include any semiconductor device including a wafer or apackaged or unpackaged bare die, as well as traditional substratesincluding circuitized boards such as printed circuit boards (PCBs). Themethod of the invention may be applied to the placement of conductivespheres 12 on any conductive site 14, whether the site, e.g. a bond pad,projects from the substrate 20 or is recessed therein. The terms“conductive site” and “bond pad” are used interchangeably herein todenote any site 14 at which a conductive sphere 12 is to be placed.

[0041] One embodiment of the sphere placement apparatus 10 and theplacement method used therewith are illustrated in drawing FIGS. 1through 4.

[0042] As depicted in drawing FIG. 1, a placement apparatus 10 forplacing a plurality of conductive spheres 12 on a substrate 20 comprisesa stencil plate or screen 30 and a sphere supply apparatus 50/50A. Thesubstrate 20 is shown with a pattern 22 of conductive sites or bond pads14 with an interpad pitch 18, wherein the pattern 22, in this example,includes all of the bond pads. The substrate 20 is shown with exemplaryregistry markers 24 by which the stencil plate 30 and substrate may beaccurately aligned to each other. The various components of theinvention may be aligned using a mechanical or pattern recognitionalignment, or any other type of accurate alignment apparatus as known inthe art.

[0043] A stencil plate 30 of the sphere placement apparatus 10 is aplanar plate with upper surface 38 and lower surface 42. An array ofthrough-holes 34 is arranged in a through-hole pattern 32 whichcorresponds to bond pad pattern 22 of the substrate 20. Through-holes 34have a diameter 36 which is slightly larger than the mean diameter 28 ofthe conductive spheres 12, so that the spheres may easily pass through,yet be closely constrained in lateral movement.

[0044] The stencil plate 30 has a thickness 40 which is configured andpositioned for holding conductive spheres 12 on bond pads 14, such thata sphere supply apparatus 50 moving across the stencil plate does notintercept the placed spheres, while preventing more than one sphere fromentering each through-hole 34.

[0045] The stencil plate 30 is configured to have its through-holepattern 32 aligned with the bond pad pattern 22. Thus, through-holes34A, 34B, 34C and 34D are shown vertically aligned by centerlines 26A,26B, 26C and 26D with bond pads 14A, 14B, 14C and 14D, respectively.

[0046] Each stencil plate 30 is configured to operate with a substrate20 having a particular bond pad pattern 22, a particular sphere diameter28, and a given range of bond pad projection height 58 (FIG. 3).

[0047] Referring to drawing FIG. 7, illustrated is a straightthrough-hole 34 of a stencil plate 30. As depicted in drawing FIG. 8,the through-hole 34 may have a beveled upper edge 72 which enhancesmovement of conductive spheres 12 into the through-hole.

[0048] The sphere placement apparatus 10 includes a sphere supplyapparatus 50 which in this embodiment is a hopper 50A having a loweropening 44 (FIG. 4) by which conductive spheres 12 may drop intothrough-holes 34 of the stencil plate 30 as the hopper is moved acrossthe upper surface 38 of the stencil plate. The hopper 50A has innerwalls 46 which contain and feed conductive spheres 12 to the stencilplate 30.

[0049] The lower opening 44 has a width 48 equivalent to about two (2)to about ten (10) sphere diameters 28. Thus, for conductive spheres 12having a diameter 28 of 1.0 mm, the lower opening may have a width 48 ofabout 0.2 cm. to about 1.0 cm.

[0050] As shown in drawing FIG. 4, the hopper 50A has a lower surface 60which is spaced from the upper surface 38 of the stencil plate 30 by ashort distance 62. Distance 62 is less than one-half (and preferablyless than one-third) of the ball diameter 28, and the stencil plate 30and hopper 50A may even be in contact. The hopper 50A is controlled toreversibly move across through-hole pattern 32 in direction 68 from afirst position 64 beyond one side of the through-hole pattern 32 to asecond position 66 beyond the other side of the pattern, droppingconductive spheres 12 into each through-hole 34, and thereby onto eachbond pad 14 directly below.

[0051] The substrate 20, stencil plate 30, and hopper 50A are eachmanipulated in robotic action to maintain the desired clearances andalignments, and to move the hopper 50A between positions 64 and 66.

[0052] In the drawings of FIGS. 1-6, the bond pads 14 of substrate 20are pictured as projecting from the substrate. The sphere placementapparatus 10 may be used for placing spheres onto recessed bond pads 14,as depicted in drawing FIG. 9. Depending on the sphere diameter 28 andthe recess depth 74 of the bond pads 14, the stencil plate thickness 40may need to be adjusted to achieve a sufficient plate-to-pad gap 56.

[0053] The hopper 50A may have inside wall surfaces 46 which aresloping, as in FIG. 4, or parallel, as in drawing FIG. 10.

[0054] Another embodiment of the sphere placement apparatus 10 is shownin drawing FIGS. 11-13. The substrate 20 and stencil plate 30 are shownas being identical to those already described above. However, the spheresupply apparatus 50 comprises a shuttle plate 80 which underlies asphere reservoir 90. Reservoir 90 may be attached to the shuttle plate80, or may comprise a separate structure. Shuttle plate 80 has an uppersurface 88 and a parallel lower surface 92, with a third pattern 82 ofthrough-holes 84. The third pattern 82 is substantially the same asthrough-hole pattern 32, although through-holes 84 may be of somewhatgreater diameter 86 than the diameter 36 of through-holes 34. Theshuttle plate 80 and sphere reservoir 90 may be configured to reversiblymove a short distance in direction 94, i.e. roughly one-half of theinterpad pitch 18. Thus, the shuttle plate 80 moves from a positionwhere its through-hole pattern 82 is non-aligned with the through-holepattern 32 (see FIG. 12) to a position where it is aligned therewith(see FIG. 13) for dropping the conductive spheres 12 into through-holes34 and thus onto the bond pads 14.

[0055] In another embodiment of the shuttle plate 80 and spherereservoir 90, they are not connected. The reservoir 90 may be kept inone position while the shuttle plate 80 moves past it for filling thethrough-holes 84.

[0056] Turning now to the method of using apparatus 10 for placingconductive spheres 12 on a substrate 20, we examine drawing FIGS. 2through 6 in sequence.

[0057] As shown in drawing FIG. 2, a step in the method of the inventioninvolves the application of a layer 52 of flux or other sticky substanceto the bond pads 14 of the substrate 20. In drawing FIG. 2, illustratedis an exemplary silk screen 54 by which the layer 52 is formed, as knownin the art. Other methods for prefluxing the bond pads 14 are alsowell-known and may be used. Any method may be used which provides asticky layer 52 to which a conductive sphere 12 will adhere. The use offlux, of course, enhances bonding of solder to a bond pad during reflow.

[0058] After a layer 52 is formed on the bond pads 14, the lower surface42 of a stencil plate 30 and the upper surface 16 of a substrate 20 arealigned to provide a desired plate-to-pad gap 56 (see FIG. 3).

[0059] The hopper 50A, having conductive spheres 12 therein, is moved indirection 68 across the through-hole pattern 32 of the stencil plate 30,whereby spheres are dropped into each through-hole 34 to become adheredto the bond pads 14 (as shown in FIG. 4).

[0060] At this point in the process, the stencil plate 30 may be tested,either visually or by other methods known in the art, to ensure that allthrough-holes 34 are filled. If any through-holes 34 are unfilled, thehopper movement may be repeated.

[0061] Upon filling of all through-holes 34 with conductive spheres 12,the substrate 20 and/or the stencil plate 30 with hopper 50A are movedin direction 70, separating the substrate as shown in drawing FIG. 5 forfurther manufacturing steps. The next step is typically one of heatingthe substrate 20 and conductive spheres 12 to cause a reflow of thesolder spheres, resulting in spheres fixed to the bond pads 14 as shownin drawing FIG. 6. Where the conductive spheres are not solder, butcomprise a metal such as germanium, the sphere placing method may beginwith solder being placed on each bond pad 14, fluxing of the soldersurface, and then placement of the conductive spheres 12.

[0062] The placement method for the embodiment of drawing FIGS. 11-13 issimilar to that of drawing FIGS. 1-4. The steps of pre-applying a layer52 of flux or sticky material to the bond pads 14, and aligning of thestencil plate 30 with the substrate 20 are the same or similar. Once theprefluxed substrate 20 is properly installed in the apparatus, theshuttle plate 80 and sphere reservoir 90 are moved from a non-alignedposition to an aligned position, whereby conductive spheres 12 fill thethrough-holes 84 of the shuttle plate and, upon reaching the alignedposition (FIG. 13), are dropped into the through-holes 34 of the stencilplate 30 and onto the prefluxed bond pads 14. The substrate 20 may bethen separated from the stencil plate 30 and the conductive spheres 12fixed by reflow to the substrate.

[0063] The methods described herein present many advantages to the BGAformation process, including higher reliability, lower cost, reducedball wastage, etc. The apparatus and methods are relatively simple, yetprovide a great deal of flexibility in substrate type, sphere size,sphere composition, etc. Non-filling of a through-hole of the stencilplate is easily cured by moving the sphere supply apparatus throughanother cycle. There is no need for using a single-head ball picker toplace a single ball as noted in the prior art.

[0064] This invention may be embodied in several forms without departingfrom the spirit of essential characteristics of the invention. Theembodiments as described herein are therefore intended to be onlyillustrative and not restrictive, and the scope of the invention isdefined by the appended claims rather than the preceding description,and all variations that fall within the metes and bounds of the subjectmatter claimed, or are equivalent thereto, are therefore intended to beembraced by the following claims:

What is claimed is:
 1. A method for placing an array of conductivespheres on prefluxed bond pads of a substrate, said method comprising:providing a substrate having a surface with a first pattern of bondpads; providing a stencil plate having upper and lower surfaces with asecond pattern of through-holes therethrough, said second pattern ofthrough-holes corresponding to said first pattern of bond pads;providing a hopper with an open top and configured to receive, hold anddispense conductive spheres as said hopper is closely moved across saidupper surface of said stencil plate; determining if each through-holecontains a conductive sphere; placing said stencil plate over saidsubstrate to align said patterns; moving said hopper across the uppersurface of said stencil plate to a position beyond said pattern ofthrough-holes to drop said conductive spheres through said through-holesonto said bond pads; and removing said substrate from adjacent saidstencil plate.
 2. The method of claim 1, further comprising: repeatingthe moving of said hopper if a through-hole is not filled.
 3. The methodof claim 1, further comprising: subjecting said substrate and saidconductive spheres to a solder reflow process.
 4. A method for placingan array of conductive spheres on prefluxed bond pads of a substrate,said method comprising: providing a substrate having a surface with afirst pattern of bond pads; providing a stencil plate having upper andlower surfaces with a second pattern of through-holes therethrough, saidsecond pattern of through-holes corresponding to said first pattern ofbond pads; providing a shuttle plate having upper and lower surfaceswith a third pattern of through-holes therethrough, said third patternof through-holes corresponding to said second pattern of through-holes;providing a sphere reservoir adjacent the upper surface of said shuttleplate with an open top and configured to receive, hold and dispenseconductive spheres into the third pattern of through-holes of saidshuttle plate; placing said stencil plate over said substrate to alignsaid first and second patterns; moving said shuttle plate across theupper surface of said stencil plate between a position aligned with saidsecond pattern of through-holes and a position non-aligned with saidsecond pattern beyond said second pattern of through-holes to drop saidconductive spheres through said second and third pattern ofthrough-holes onto said bond pads; determining if each through-holecontains a conductive sphere; and removing said substrate from adjacentsaid stencil plate.
 5. The method of claim 4, further comprising:repeating the moving of said shuttle plate if a through-hole is notfilled.
 6. The method of claim 4, further comprising: subjecting saidsubstrate and said spheres to a solder reflow process.
 7. The method ofclaim 4, wherein the through-holes of said stencil plate have beveledupper edges.
 8. The method of claim 4, wherein the side walls of thehopper taper in toward the lower opening.
 9. The method of claim 4,wherein the through-holes of said stencil plate have beveled upperedges.
 10. The method of claim 4, wherein the inner edges of the spherereservoir are oriented so as to extend from the shuttle plate upward atan obtuse angle.